A preferred method for the production of peptides, particularly for pharmaceutical use, is through genetic engineering. However, the production of peptides by recombinant technologies has specific limitations. For example, because the genetic code does not include non-naturally occurring L-amino acids, D-amino acids, radioactive amino acids, and other detectable labels, production of recombinant peptides bearing such modifications is difficult.
In addition, natural amino acid modifications such as C-terminal amide group substitution, performed routinely in vivo, are difficult to perform in vitro. Because these post-translational modifications often result in the most potent or longest acting form of the peptide, they are the form most needed and desired for pharmaceutical use. For many peptides, C-terminal amidation is important for biological activity. However, recombinant expression systems for large scale production of active peptides cannot easily carry out the necessary C-terminal modification.
Carboxypeptidase enzymes are known to catalyze transpeptidation reactions, yielding C-terminally amidated peptides. However, wild type carboxypeptidase enzymes are not useful for C-terminal amidation of many peptides. For example, the inherent substrate specificity of wild-type carboxypeptidase restricts the variety of peptides that may be modified using this enzyme.
In particular, carboxypeptidase Y displays a strong preference for peptides with a penultimate apolar residue. Substrates having a penultimate amino acid with a positively charged side chain are not effectively transacylated by carboxypeptidase Y. For example, the substrate FA-Arg-Ala-OH is hydrolyzed about 500 times more slowly than the substrate FA-Leu-Ala-OH. Unfortunately, the amino acid sequences of many pharmaceutically important peptides, including growth hormone releasing factor (GRF) or glucagon like peptide (GLP1), have a penultimate or ultimate amino acid with a positively charged side chain, making transamidation with carboxypeptidase Y commercially impractical.
Although several mutant carboxypeptidases are known to have enhanced hydrolysis activity with a variety of peptide substrates, the study of numerous mutant carboxypeptidase enzymes demonstrates that there is no specific correlation between mutants having enhanced hydrolysis activity and mutants having enhanced transamidation activity. For example, although the carboxypeptidase S1 from Penicillium janthinellum is capable of efficiently hydrolyzing peptide substrates with a basic P.sub.1 residue, the enzyme does not efficiently transpeptidate them. Breddam, Carlsberg Res. Commun., 53(5):309-320 (1988).
It would be very useful to provide a modified carboxypeptidase having improved activity for transacylation of peptide substrates whose sequence includes a penultimate amino acid (P.sub.1) having a positively charged side chain.